The syphilis reactor grid: help or hindrance for syphilis surveillance?

Evaluation of Automated Processing of Electronically Reported Serological Tests for Syphilis Using Current and Historical Syphilis Results Compared With Traditional Reactor Grid Processing in Florida

BACKGROUND

Syphilis in Florida increased 49% from 2016 to 2020. Moreover, many serological tests for syphilis (STS) do not indicate current infection. Traditionally, syphilis surveillance systems used reactor grids, a method for prioritizing STS for investigation based on age, nontreponemal titer, and/or sex. In 2022, Florida's sexually transmitted disease surveillance system implemented an automated method for processing electronically reported STS (eSTS), expanding upon the reactor grid, using an individual's current STS (treponemal and nontreponemal), treatment history, and historical STS results aiming for more efficiently processing eSTS. We compared the new method of processing eSTS results against the reactor grid and determined potential value in time/cost savings of this change.

METHODS

All eSTSs (n = 4144) from January 2, 2023 to January 8, 2023, were compared by how the logic-based method processed test results versus how the reactor grid processed test results. Each method was compared using measurements of accuracy (e.g., sensitivity/specificity). Time and cost savings in eSTS processing were estimated.

RESULTS

Using the surveillance case definition as reference, the accuracy of the logic-based method for processing eSTS was nearly double (82.3% vs. 43.6%), had greater specificity (79.0% vs. 33.0%), and increased positive predictive value (47.5% vs. 22.0%) when compared with the reactor grid method. Sensitivity (99.5% vs. 98.6%) and negative predictive value (99.9% vs. 99.2%) remained similar. The logic-based method is estimated to save 7783 hours annually (~$185,000).

CONCLUSIONS

Processing eSTS based on current and historical STS results is significantly more accurate than using a reactor grid. Moreover, these improvements save time and resources that can be better allocated to other program prevention activities.

An Automated Syphilis Serology Record Search and Review Algorithm to Prioritize Investigations by Health Departments

BACKGROUND

Reactive syphilis serologies are investigated by health departments to determine if they represent new infection, reinfection, or treatment failure. Serologies prioritized for investigation based on nontreponemal test titer and age (using a "reactor grid") undergo manual record search and review. We developed a computerized algorithm that automates the record search and review.

METHODS

We developed and tested the algorithm using a Florida Department of Health data set containing serologies reported January 2016 to December 2018 and previous records linked to each individual. The algorithm was based on the syphilis case definition, which requires (except primary cases with signs and symptoms) (1) a positive treponemal test result and a newly positive nontreponemal test result or (2) a 4-fold increase in nontreponemal test titer. Two additional steps were added to avoid missing cases. New York City Department of Health and Mental Hygiene validated this algorithm.

RESULTS

The algorithm closed more investigations (49.9%) than the reactor grid (27.0%). The algorithm opened 99.4% of the individuals investigated and labeled as cases by the health department; it missed 75 cases. Many investigations opened by the algorithm were closed by the reactor grid; we could not assess how many would have been cases. In New York City, the algorithm closed 70.9% of investigations, likely because more individuals had previous test in the database (88.2%) compared with Florida (56.5%).

CONCLUSIONS

The automated algorithm successfully searched and reviewed records to help identify cases of syphilis. We estimate the algorithm would have saved Florida 590 workdays for 3 years.

Estimates of the Prevalence and Incidence of Syphilis in the United States, 2018

BACKGROUND

Syphilis is a genital ulcerative disease caused by the bacterium Treponema pallidum that is associated with significant complications if left untreated and can facilitate the transmission and acquisition of HIV infection. The last prevalence and incidence estimates of the burden of syphilis in the United States were for 2008.

METHODS

We generate syphilis prevalence and incidence estimates for 2018 among adults aged 14 to 49 years. We fit a simple mathematical model to 2018 case report data to generate 10,000 sets of estimates for age and sex subpopulations and summarize our estimates by their median (50th percentile); uncertainty intervals are characterized by their 25th (Q1) and 75th (Q3) percentiles. We also used our methodology to reestimate 2008 prevalence and incidence estimates.

RESULTS

In 2018, there were an estimated 156,000 (Q1, 132,000; Q3, 184,000) prevalent and 146,000 (Q1, 126,000; Q3, 170,000) incident syphilitic infections in people aged 14 to 49 years. Men accounted for roughly 70% of prevalent infections and more than 80% of incident infections. In both sexes, there were more prevalent and incident infections in 25- to 49-year-olds than 14- to 24-year-olds. Using these methods to reanalyze 2008 data, syphilis prevalence and incidence estimates have increased 164% and 175%, respectively, between 2008 and 2018.

DISCUSSION

Although not as common as other sexually transmitted infections, syphilis should be monitored because of its devastating sequelae. As it continues to increase in frequency, it will be important for future work to continue to track its trajectory and burden.

Reactor Grids for Prioritizing Syphilis Investigations: Are Primary Syphilis Cases Being Missed?

BACKGROUND

Health departments prioritize investigations of reported reactive serologic tests based on age, gender, and titer using reactor grids. We wondered how reactor grids are used in different programs, and if administratively closing investigations of low-titer tests could lead to missed primary syphilis cases.

METHODS

We obtained a convenience sample of reactor grids from 13 health departments. Interviews with staff from several jurisdictions described the role of grids in surveillance and intervention. From 5 jurisdictions, trends in reactive nontreponemal tests and syphilis cases over time (2006-2015) were assessed by gender, age, and titer. In addition, nationally-reported primary syphilis cases (2013-2015) were analyzed to determine what proportion had low titers (≤1:4) that might be administratively closed by grids without further investigation.

RESULTS

Grids and follow-up approaches varied widely. Health departments in the study received a total of 48,573 to 496,503 reactive serologies over a 10-year period (3044-57,242 per year). In 2006 to 2015, the number of reactive serologies increased 37% to 169%. Increases were largely driven by tests for men although the ratios of tests per reported case remained stable over time. Almost one quarter of reported primary syphilis had low titers that would be excluded by most grids. The number of potentially missed primary syphilis cases varied by gender and age with 41- to 54-year-old men accounting for most.

CONCLUSIONS

Reactor grids that close tests with low titers or from older individuals may miss some primary syphilis cases. Automatic, computerized record searches of all reactive serologic tests could help improve prioritization.

Going Off Grid: Modeling an Automated Record Search to Process Electronically Reported Reactive Nontreponemal Syphilis Tests

BACKGROUND

Before searching prior records, sexually transmitted disease programs use syphilis reactor grids to exclude some reactive nontreponemal test results (RNTs) based on patient age, gender, and test titer. We propose a new algorithm that starts with comparing RNTs to previous syphilis nontreponemal tests and current treponemal test results.

METHODS

Deduplicated RNTs from Florida's surveillance system (2006-2015) were extracted and stratified on morbidity. An algorithm was developed to triage RNTs. Sensitivity and specificity of the algorithm and the current reactor grid were estimated using reported syphilis cases. A random sample of cases missed by the proposed algorithm, stratified by stage of disease, was reviewed to verify case classification.

RESULTS

Reported RNTs increased 58% from 2006 (n = 34,808) to 2015 (n = 55,001) (total = 372,902). The current reactor grid removed 91,518 (24.5%) RNTs and missed 1149 potential cases. Strictly following the reactor grid would result in a sensitivity of 97.4% and a specificity of 27.5%. The proposed algorithm would remove 242,078 (64.9%) RNTs and miss 2768 potential cases. This results in a slightly lower sensitivity of 93.8%, but nearly triples the specificity, 72.9%. A review of a random sample of the 2768 cases estimated that 72.7% would not have met the syphilis surveillance case definition, resulting in an adjusted sensitivity of 98.4%.

CONCLUSIONS

In Florida, an algorithm that starts by searching previous syphilis test results vastly improved specificity and slightly improved sensitivity compared with the current reactor grid. Implementing an automated algorithm could increase case ascertainment efficiency and further prioritize likely cases for investigation.

A Web Application to Facilitate Syphilis Reactor Grid Evaluations

BACKGROUND

Many health departments use a "reactor grid" to determine which laboratory-reported syphilis serologic test results require investigation. We developed a Web-based tool, the Syphilis Reactor Grid Evaluator (SRGE), to facilitate health department reactor grid evaluations and test the tool using data from Seattle & King County, Washington.

METHODS

We developed SRGE using the R Shiny Web application framework. When populated with a data set including titer results and final disposition codes, SRGE displays the percent of verified early syphilis cases by serologic titer result and patient age in each cell of the grid. The results can be optionally stratified by sex, test type, and previous rapid plasma reagin titer. The impact of closing laboratory results without investigation in cells selected by the user is dynamically computed. The SRGE calculates the percent of all laboratory reports closed ("efficiency gained"), the proportion of all early syphilis cases closed without investigation ("case finding loss"), and the ratio of percent of cases identified for investigation to percent of all laboratory reports investigated ("efficiency ratio"). After defining algorithms, users can compare them side-by-side, combine subgroup-specific algorithms, and export results. We used SRGE to compare the current Public Health-Seattle & King County (PHSKC) reactor grid to 5 alternate algorithms.

RESULTS

Of 13,504 rapid plasma reagin results reported to PHSKC from January 1, 2006, to December 31, 2015, 1565 were linked to verified early syphilis cases. Updating PHSKC's current reactor grid could result in an efficiency gain of 4.8% to 25.2% (653-3403 laboratory reports) and case finding loss of 1% to 8.4% (10-99 fewer cases investigated).

CONCLUSIONS

The Syphilis Reactor Grid Evaluator can be used to rapidly evaluate alternative approaches to optimizing the reactor grid. Changing the reactor grid in King County to close more laboratory results without investigation could improve efficiency with minimal impact on syphilis case finding.